This invention relates to an optical triangulation ranging system, and more particularly to a non-contact sensor and camera system that has a variable geometry and is reconfigurable in real time to provide a requested performance.
Range from a reference point or plane to a given target point can be measured in a variety of ways. Passive techniques, such as stereo or range-from-focus, require illuminated target surfaces and typically require complexly patterned surfaces for reliable results. Active echoing techniques, such as RADAR, SONAR and LIDAR employ emitted electromagnetic, acoustic, and light energy, and monitor the reflected energy from the target surface. These techniques use a time-of-flight measurement as a basis for determining range, and are typically electronically expensive and complex. In machine vision research and application, a different active technique called structured illumination has been employed. A ray of light is directed to the target surface along a direction which is not coaxial with the optic axis of the one or two dimensional sensing device. The intersection of the ray and target surface produces a spot of light which is imaged on the sensing plane of the imaging device. The 3-D position of this spot in space may be calculated from the known position and orientation of the imaging device from basic trigonometric relations. This structured illumination technique is called triangulation.
Simple laser triangulation devices have proliferated in recent years and behave nominally as shown in FIGS. 1a and 1b. With the geometry of these figures, where a laser 10, lens 11, detector 12 and object 13 are illustrated, the reflected spot is imaged higher on the detector 12 as the target surface moves farther from the imaging lens. In most practical devices using this geometry, however, the reflected spot is in focus for only one target distance and is blurred in varying degrees for all other distances in the depth of field as shown in FIGS. 2a and 2b. This has the effect of reducing the sensitivity of the device and effectively reduces the depth of field.
Three of the basic performance measures of a ranging system are standoff distance, depth of field, and range resolution. The standoff distance is the nominal range of the device and is typically arbitrarily chosen as the near point, mid point or far point in the depth of field, which is the full extent of range values the system is capable of measuring. The resolution is the smallest range value the system can discriminate, and typically varies over the depth of field. Triangulation devices are engineered for a certain standoff distance, depth of field, and range resolution. That is, the geometry of the devices is chosen based on the application, and desired changes in performance require changing the imaging optics. In certain applications, such as imaging in remote areas, such inflexibility is unacceptable.
The first problem to be solved is how to provide blur-free imaging on the detector, so that all reflected target returns are in focus. The second problem is how to construct a sensor that is reconfigurable in the sense that standoff distance, depth of field, and/or range resolution can be varied under electronic control without requiring the replacement of components.
It is well known in the photographic industry that tilting the camera at the moment of exposure leads to an effect called keystoning, in which parallel lines in the subject appear as converging lines in the result. This can be rectified during printing by tilting the enlarger easel by the same angle by which the camera is tilted. To achieve sharp imagery throughout the result, the enlarger lens must also be tilted slightly, so that the planes of the lens, easel, and negative all meet at a common location, in accordance with what is known as the Scheimpflug condition in optics. The Scheimpflug condition can also be interpreted in an alternative way: a line on the object side of the lens is imaged to a line on the image side of the lens, and the two lines intersect with the line representing the plane of the lens, as shown in FIG. 3a. If the ray of light for the system is directed along some line on the object side of the lens, then we should place the detector along the Scheimpflug condition - predicted line on the image side to guarantee blur-free imaging of reflected target returns as shown in FIG. 3b.